1. Field of the invention
The present invention relates to a single crystal quartz thin film and a preparation thereof. The single crystal quartz thin film (or single crystal rock crystal thin film) of the present invention can be used for an oscillator, a vibrator, a surface elastic wave element for a high frequency filter, a light waveguide and the like.
2. Description of Related Art
Quartz (or rock crystal) has a low temperature (&lt;573.degree. C.) phase of silicon dioxide. However, when a quartz-type backbone basically forming a quartz structure has a temperature of at least 870.degree. C., it is unstable. A melting point of silicon dioxide is 1730.degree. C. which is remarkably higher than 870.degree. C. Since a cristobalite-type crystalline structure is stable at a temperature near the melting point, it is impossible to prepare quartz by a simple high temperature treatment.
A conventional method of preparing quartz is only a hydrothermal synthetic method comprising growing a single crystal quartz on a seed crystal from an alkaline solution of silicon dioxide with providing a temperature difference at a high temperature and high pressure. This method of preparing quartz is described in Ceramics Japan, 15 (1980), No. 3, pages 170-175.
According to the above document, a hydrothermal method for preparing quartz comprises charging an aqueous solution (for example, 0.1 to 1 N aqueous NaOH or Na.sub.2 CO.sub.3 solution) in a large autoclave, positioning a starting material of waste quartz at a lower part of the autoclave and a seed crystal on a convection control panel at an upper part of the autoclave, and heating the closed autoclave by an external heater. At this time, by maintaining the temperature of the upper part of the autoclave at 330-340.degree. C. and the temperature of the lower part of the autoclave at 360-370.degree. C. to give a constant temperature difference and maintaining an internal pressure of the autoclave at a high pressure of 800 to 1,500 kg/cm.sup.2, the starting material of waste quartz is molten and transported to the upper part by the convection, the waste quartz is supersaturated in the upper part having lower temperature than the lower part so that quartz is deposited and grows on the seed crystal.
The hydrothermal synthesis can give only a large bulk crystal or particulate powder. Presently, when a thin film is necessary for an oscillator, a vibrator, and a surface elastic wave element for a high frequency filter, the large single crystal prepared by the hydrothermal synthesis is cut to give the thin film.
However, with the higher frequency of the recent telecommunication, it is necessary to obtain a thinner quartz. For example, Japanese Patent Kokai Publication No. 327383/1993 proposes that quartz is adhered to a semiconductor substrate, polished and processed to give a thin film. However, the preparation of thin film by the processing has the limitation of the thickness of the processed thin film and has a high cost.
The conventional method of preparing quartz by the hydrothermal procedure needs a large-scale apparatus to use the high pressure, for example, an autoclave and can give a lower cost only by the growth of a large single crystal by a huge apparatus. Since this procedure cannot easily give a arbitrary shape of quartz single crystal, the large single crystal is processed to give quartz having the desired shape.
Particularly, an oscillator, a vibrator and a surface elastic wave element which are a main use of quartz require a thinner quartz with the higher frequency of the telecommunication. However, the conventional method of forming a quartz thin film by the cutting of the large single crystal can practically give the thickness of at least 50 .mu.m and cannot satisfy the requirement of thinner film.